1. Field of the Invention
The present invention relates to a color processing apparatus which calculates a calorimetric value under a given viewing illuminant and a method thereof.
2. Description of the Related Art
With the prevalence of personal computers, a large number of people have started to use image input devices such as a digital cameras, image scanners, and the like. Opportunities have increased to input an image by an image input device, to display the image on an image display device such as a cathode-ray tube (CRT), liquid crystal panel (LCD), or the like, and to output the image using an image output device such as a printer or the like. In this case, a color matching process (gamut mapping) is necessary to correct differences in color appearance caused by differences in color gamut between the image input device, image display device, and image output device.
A color matching process is a process for associating, the colors of the respective devices with each other, based on a device profile which describes color reproducing characteristic data of the respective devices subject to color matching. For example, in case of a printer, color reproducing characteristic data is data obtained by measuring the color of a color patch output by the printer.
A color matching process can absorb differences in color appearance between devices. However, when an illuminant (to be referred to as a viewing illuminant, hereinafter) under which an image is viewed is different from an illuminant (to be referred to as a colorimetric illuminant, hereinafter) upon color measurement for obtaining color reproducing characteristic data, colors cannot be matched. In other words, colorimetric measurement needs to be done under a viewing illuminant to obtain color reproducing characteristic data to be described in a device profile.
As a method of obtaining a colorimetric value under a viewing illuminant, for example, a technique disclosed in Japanese Patent Laid-Open No. 2002-218266 is available. This technique measures a color patch output by a device subject to color matching by using a spectrophotometer, and measures a viewing illuminant by using a spectral radiance meter. Then, a tristimulus value of CIEXYZ is calculated from the spectral reflectance of the color patch and the spectral radiance of the viewing illuminant, and is described in a device profile.
According to this technique, a colorimetric value under a viewing illuminant can be accurately calculated. However, the size of spectral reflectance data of color patches is too large to hold in a memory, device profile, or the like.